I would suggest a short period of vernalization and/or a
"dash" of a little gibberellic acid. What we have done in
the past is to harvest seeds from the plants of interest and
allow for the seeds to dry completely (approximately 1-2
weeks at room temperature) and then store the seeds at 4
The seeds are taken from the cold and placed on the top of
throughly moistened soil, covered with a layer of plastic
wrap (saran wrap) and allowed to germinate at 22-25 degrees.
The "dash" of GA helps with seeds which require a
"jump-start" and seems to help with the most stubborn lines.
Hope this info will be of use to you.
Chu at mis.barnard.columbia.edu